The invention relates generally to a method of copying a colored original.
More particularly, the invention relates to a method of electronically enhancing the image of a colored original.
In a known method of electronically processing the image of an original, the original is electrooptically scanned along rows and columns in each of the three primary colors red, green and blue. The resulting imaging signals are converted into a luminance signal and two chrominance signals representing differences between colors. The luminance signal is processed for contrast enhancement by dividing such signal into two portions one of which is sent through a high-pass channel and the other of which is sent through a low-pass channel having different amplifying characteristics than the high-pass channel. The two portions of the luminance signal are added after passing through the respective channels to thereby yield an enhanced luminance signal.
The transformation of imaging signals into luminance and chrominance signals is used principally in the video art. The primary purpose is to carry out color corrections electronically when color distortion, e.g., a color cast, is present in the motif of the original or when color distortions requiring correction arise during transmission. Electronic image processing is frequently used to optimally adjust the color saturation and the color contrasts of an image to the characteristics of the photographic copy material or to accentuate the color saturation and the color contrasts of the original. The principles of electronic color correction are described, for example, in the following texts: (i) H. Lang, Farbmetrik und Farbfernsehen, pp. 326-334 and 431 ff., R. Oldenbourg Publishing House, Munich, Vienna (1978); and (ii) W. K. Pratt, Digital Image Processing, pp. 50-90 and 155-161, John Wiley & Sons, New York/Chichester/Brisbane/Toronto (1978).
In electronic image processing, contrast treatment of the luminance signal is performed in order to adjust the gradation of the entire transmission system to the photographic copy material (global contrast treatment) and to accentuate the contrast in specific regions of the original (local contrast treatment). This allows an increase in the sharpness of the image to be obtained. The principles of these procedures are described in detail in the following texts: (i) F. M. Wahl, Digitale Bildverarbeitung, Springer Publishing House, Berlin, Heidelberg, New York, Tokyo (1984); and (ii) W. K. Pratt, Digital Image Processing (see above).
Electronic color processing using color correction circuits is being increasingly employed in the reproduction of colored originals (positive-positive) and the production of colored photographic positive images from colored negatives. See, for instance, the European patent application Nos. 70 680; 131 430; and 168 818. The original is here scanned along rows and columns and the resulting imaging signals are modified in accordance with specific criteria. The imaging signals obtained from an original are generally digitized and can then be stored in a digital memory. As a rule, scanning of the original is performed serially in the three primary colors red, green and blue (the primary colors red, green and blue will hereafter also be denoted by the letters R,G and B, respectively).
Experience has shown that the following parameters of an image must be adjustable in order to produce an optimum positive image:
(a) Color balance.
(b) Color saturation.
(c) Contrast (Gradation).
Frequently, these parameters cannot be adjusted independently of one another. Thus, when the gradation is changed, the color saturation, for example, may also be shifted. For this reason, the video art converts the RGB imaging signals into a luminance or brightness signal and two chrominance signals containing only the color information. However, it has been found that the contrast treatment in the luminance channel can influence the color saturation in the chrominance channels. Such shifting of the color saturation cannot be tolerated when strict requirements are imposed on image quality. In particular, it has been observed that an increase in brightness (greater amplification of the luminance signal) is accompanied by a desaturation of the image while the image appears to be more highly saturated when the brightness is decreased.
It has further been found that adjustment of the color saturation becomes critical when an original contains weakly saturated regions and more strongly saturated regions next to one another. Thus, the modulation rang of the chrominance channels has a limit which should not be exceeded in practice. Nevertheless, it may happen that this limit is reached or exceeded when the color saturation in one or both channels is increased. This over modulation leads to an undesired change in the nature of the colors, that is, to color falsifications.